1. Field of the Invention
The present invention relates to the structure of a semiconductor device and a method for manufacturing the same. In particular, the present invention relates to an interconnection structure including barrier metal made of a copper film and a method for manufacturing the same.
2. Description of the Related Art
In multi-level wiring of semiconductor integrated circuits (LSIs), copper (Cu) films having a low resistivity are used. Damascene wiring that trenches and via holes formed in an inter-level insulating film is filled with a Cu film is in the mainstream. Interconnection widths are being decreased with reductions in the sizes of LSIs, and interconnection thickness are being decreased in order to reduce capacitances between interconnections. Accordingly, in a fine damascene interconnection, the proportion of a barrier metal film having a high resistivity in the cross-sectional area of the interconnection greatly influences the resistance of the interconnection. In other words, the resistance of the damascene interconnection decreases as the thickness of the barrier metal film decreases. However, the prevention of diffusion of Cu atoms into an inter-level insulating film, adhesion to a Cu film, and adhesion to the inter-level insulating film are simultaneously required for the barrier metal film.
In particular, adhesion between the barrier metal film and the Cu film is very important for the electromigration (EM) resistance and stress migration (SM) resistance of the interconnection. Further, it is desired that the barrier metal film be formed to have the smallest film thickness satisfying the above-described requirements, to have a uniform thickness on the bottom surfaces and side surfaces of trenches formed in the inter-level insulating film, and to be conformal.
Recent circumstances of the formation of a thin barrier metal film will be described. Ordinary physical vapor deposition (PVD) has poor step coverage. Accordingly, in a dual damascene structure formed by filling trenches and via holes formed in an inter-level insulating film with metal, it is difficult to form a conformal barrier metal film by PVD. Thus, ionized PVD has been developed in which metal ions are introduced by a substrate bias to improve bottom coverage and in which side coverage is improved using the resputtering effect of ions. The ionized PVD has been used to form a barrier metal film. However, conformal film formation adequate for ensuring interconnection resistance, barrier performance, and adhesion will become increasingly difficult due to reductions in interconnection widths and increases in the aspect ratios of interconnections in the future. Although a conformal barrier metal film can be formed by chemical vapor deposition (CVD), a high-temperature process cannot be applied to a wiring process because of the problem of SM failures. Accordingly, CVD has the problem that there are few kinds of source gases of material for forming a barrier metal film which are decomposed at temperatures allowable in the wiring process.
As a method for forming an extremely thin conformal film, atomic layer deposition (ALD) has been proposed in which a thin film is grown by stacking atomic layers on a surface of a substrate one by one. ALD is not suitable for the formation of a thick film but can form an extremely thin film with good step coverage. Similar to CVD, ALD has the problem that it is difficult to thermally decompose source gas at temperatures within the range allowable for a wiring process. Thus, there have been proposed a method in which decomposition is promoted by plasma irradiation in order to lower the temperature of a wiring process in the process of decomposing adsorbed source gas, and a method in which decomposition is promoted by UV light irradiation.
For recent inter-level insulating films, low-dielectric constant insulating films are used in order to reduce signal delays. Such low-dielectric constant insulating films include not only organic insulating films but also inorganic insulating films which contain a large amount of carbon (C) and have many vacancies and in which oxidizing species such as water (H2O) are trapped. Accordingly, in a method in which the temperature of film formation is lowered using plasma irradiation in CVD or ALD, carbon in an insulating film is emitted by plasma irradiation in the process of decomposing source gas, whereby the insulating film is damaged. In particular, in the case of plasma using gas containing hydrogen (H) or oxygen (O), a low-dielectric constant insulating film is etched, and the insulating film may be delaminated.
In a low-dielectric constant insulating film containing a large amount of oxidizing species, there are cases where a barrier metal film is oxidized during the formation of the barrier metal film even if ALD or CVD is used in which decomposition temperature is lowered in an auxiliary manner using plasma irradiation or UV light irradiation. An oxidized barrier metal film cannot prevent oxidizing species from passing therethrough. As a result, the entire barrier metal film is oxidized by oxidizing species, whereby adhesion between the barrier metal film and wiring material such as a Cu film is lowered.
Adhesion between the barrier metal film and the Cu film includes adhesion depending on material and adhesion which changes with time due to a change in the quality of the barrier metal film. In particular, a change in adhesion with time causes an SM or EM failure or the like not only during a manufacturing process but also in actual use. Carbon-containing molecules in an insulating film are emitted in a process or an insulating film curing process involving plasma irradiation, electron beam (EB) irradiation, or ultraviolet (UV) light irradiation. As a result, the insulating film is damaged, and water is easily adsorbed on sites to which leaving carbon has bonded.
Causes of a change in the quality of the barrier metal film with time during a manufacturing process or in the actual use of an LSI include a decrease in adhesion between the barrier metal film and the Cu film which is caused by the oxidation of the barrier metal film with oxidizing species contained in the insulating film. Further, the carbonization (change to carbide) of the barrier metal film with C-containing molecules contained in the insulating film is also a cause of a change in the quality of the barrier metal film with time.
In the future, it will become increasingly difficult to prevent a change in the quality of the barrier metal film and to ensure adhesion between the barrier metal film and the Cu film. There has been proposed a process in which oxide is formed beforehand at the interface between the insulating film and the barrier metal film. However, in the case where oxide is actively formed, oxide having a large valence and a low concentration is formed. Thus, a desired form cannot be obtained.